Shoulder fractures are not uncommon, especially among senior citizens with osteoporosis. In certain instances, shoulder replacement surgery is necessary. One type of fracture that commonly requires shoulder replacement surgery is called a four-part fracture. Such a fracture is characterized by the shoulder being broken into four parts—the lesser tuberosity, the greater tuberosity, the humeral head, and the humeral shaft.
During a shoulder replacement operation (also known as shoulder arthroplasty surgery), a canal found within the humeral shaft is used for receiving the humeral component of the shoulder prosthesis. If necessary, in order to better accommodate the humeral component, the canal may be enlarged using known methods. The humeral component of the prosthesis typically includes a stem and a head, which is a partial sphere that replaces the fractured humeral head. The stem and the head may be two separate components, or they may be a single component. Regardless of whether the humeral component is a single part or two parts, a collar is formed between the stem and the head. Between this collar and the fracture line on the proximal end of the humerus, the fractured lesser and greater tuberosities are normally anchored around the stem of the humeral component, as closely as possible to their original positions.
Before implanting the permanent stemmed humeral component, a temporary component, often called a humeral stem provisional, is temporarily inserted into the humerus to determine the proper placement of the stemmed humeral component. Proper positioning of the final stemmed humeral component is very important. Improper positioning can result in a range of problems from mild discomfort to severe pain, as well as a lack of proper range of motion, depending upon the degree and type of improper positioning. Accordingly, it is important that the humeral stem provisional be properly positioned, and that such proper position can be reliably replicated when implanting the final stemmed humeral component. The two key areas of proper positioning are the stemmed component's axial position and its retroversion.
Currently, several devices are available to assist in properly positioning the stemmed humeral component within the humerus. For example, Tornier® markets a system under the Aequalis® name that includes a jig that uses a portion of the anatomy for referencing the position of the humeral implant. However, this jig is relatively large and bulky, and it is difficult to perform a range of motion analysis with the jig in place. Other smaller systems, such as the DePuy® Global™ Fx System also include a positioning jig. Although the jig of the Global™ Fx System allows for better range of motion analysis than the Tornier® system, the Global® Fx System still includes a somewhat large positioning jig, which could benefit from being changed to a more low profile system. Thus, an improved system for properly positioning the stemmed humeral component is desired.